In the petrochemical industry, smaller olefins are often produced from a precursor feedstock by a thermal cracking process. This cracking process involves heating up a precursor feedstock comprising larger hydrocarbons. As a result of the increased thermal energy, carbon bonds present in the precursors feedstock will be broken, thereby turning the long-chains of hydrocarbon molecules into shorter, smaller ones. The necessary temperatures to initiate the cracking process may reach up to a thousand degrees, depending on the supplied hydrocarbon and the desired cracked end-product. Usually, other cracking process conditions such as residence time, dilution, flow, pressure, etc., may be tuned to achieve the highest possible yield.
Commonly, the steam cracking process takes place inside a reactor suspended in a large, gas-fired furnace which heats up the reactor from the outside. Often tens to hundreds of these reactors are stacked together in one furnace to increase production capacities up to several thousand kilotons per year (kta). These reactors, sometimes called cracking tubes, are fabricated in many different shapes and sizes in an effort to increase the capacity, improve the selectivity, yield, and/or thermal efficiency of the process.
During steam cracking the precursor feedstock supplied in gas-state flows to the reactor at a high velocity where it is diluted with steam and heated without the presence of oxygen. However, as a result of this process a deposition of coke, i.e., a solid residue composed of carbon, may build up on the inner wall of the reactor. This deposition of coke has several adverse effects on the productivity of these reactors:
(1) Coke has a low thermal conductivity, so deposition of coke may lower the thermal efficiency of the system which will in turn require the fuel flow rate to be increased to maintain the same level of production, thus further increasing the coke deposition rate.
Moreover, different coke deposition rates across a series of reactors suspended in a common furnace will prevent proper temperature control needed to maintain desired production selectivity. The low thermal conductivity of the cokes layer also results in higher tube metal temperatures, which may reach the design limits of the alloy that is used.
(2) Sustained deposition of coke may decrease the cross-sectional area of a reactor available for the feedstock gas resulting in a higher process gas velocity and a higher pressure drop over the reactor. To compensate for this pressure drop, the overall pressure inside the reactor will have to be increased, which inadvertently leads to reduced process selectivity towards light olefins because of an increased rate of secondary reactions between those olefins.(3) Presence of coke decreases the carbon yield of the cracking process since all the carbon atoms that would otherwise be collected as light olefins are instead incorporated into the coke and are hence lost.
To limit the adverse effects of coke deposition over time a regular shutdown is required to decoke the reactors. The decoking process typically involves taking a whole furnace offline for 1 or more days to oxidize coke depositions and remove them from the inner wall of all the reactors. Consequently, a decoking process drastically decreases the productivity of a furnace by interrupting the run-length and increases the production costs, by stacking material and energy costs needed to perform the decoking process and thereafter restart the cracking process. Furthermore, given the exothermic nature of decoking, thermal damage may occur to the reactors during decoking.
Internal fins such as described in GB969796 may achieve improved heat transfer by increasing the internal surface area. As the exposed reactor surface area increases, however, so does the laminar flow layer in contact with the reactor wall. In this layer, high residence times are coupled with high temperatures, giving rise to significant losses of valuable product yields. Additionally, more of the reactor wall is exposed for formation of coke deposits. Hence, while the thermal performance of the tubes may be improved, the effect on coke formation and run lengths can even be negative.
U.S. Pat. No. 5,950,718 aims to resolve these issues by adding convex elements to the reactor wall in order to break up the laminar layer and promote turbulent mixing. While such devices may improve heat transfer coefficient, they typically suffer from excessive drag, as potential energy in the form of pressure is transformed into turbulent kinetic energy. Additionally, such obstructions in the flow induce recirculating flow patterns with locally high residence times which are prone to coke formation. The separate welding of each of the elements also adds an additional maintenance risk, as elements can break off because of the high local thermal and shear stresses they are exposed to.
Accordingly, there is a need for new technologies that allow for an increase of the run-lengths of steam cracking reactors and furnaces. There is also a need for technologies that aim to improve the olefin selectivity during the thermochemical process. There is also a need for technologies that aim to improve heat transfer. There is also a need for technologies that aim to decrease thermal stress. There is also a need for technologies that aim to operate at a higher severity by increasing the maximal process gas temperature. There is also a need for technologies that aim to operate at a higher throughput by increasing the maximum load. There is also a need for technologies that aim to limit the occurrence and/or magnitude of a pressure drop.